Surface wave converter with optimized reflection

ABSTRACT

A transducer includes base cells each having a length λ, where λ is a wavelength of a center frequency of the transducer. The base cells are arranged in a direction of propagation of a surface acoustic wave. The base cells include exciting base cells and reflecting base cells. The exciting base cells each have a same number of electrode fingers. Each base cell has a reflection portion. The reflection portion of each base cell has a same value mRo, where m is one of −2, −1, 0, 1, or 2 and Ro is a reference reflection. Each non-zero reflection portion in each base cell has a same phase position. Excitation and reflection in the transducer are equal in phase in one direction of propagation of the surface acoustic wave and opposite in phase in an opposite direction of propagation of the surface acoustic wave.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to PCT Application No. PCT/DE01/00630,filed on Feb. 19, 2001, and to German Patent Application No. 100 10089.9, filed on Mar. 2, 2000.

BACKGROUND

This invention relates to an interdigital transducer used to producesurface acoustic waves (SAW). Such an interdigital transducer is calleda surface acoustic wave transducer, and is referred to hereinafter assimply a “transducer”. A transducer of this type includes twocomb-shaped electrodes, each called a current-collecting bar or bus bar,which comprise electrode fingers. Two such combined comb-shapedelectrodes form the interdigital transducer. A surface acoustic wavefilter can, for example, be made up of a piezoelectric substrate withtwo interdigital transducers that act as an input and an outputtransducer. The acoustic surface wave produced in the input transduceris converted back to an electrical signal in the output transducer. Thepath taken by the acoustic surface wave, which can, if necessary, bebounded on both sides of the transducers by reflectors, or can penetrateinto them, is also called an acoustic track. The efficiency of theelectroacoustic conversion is optimum at the center frequency. Throughvarious design and circuit modifications, the filter is adjusted suchthat it has a good passband response over a desired bandwidth near itscenter frequency. Within this band, a filter should have as low aninsertion loss as possible, i.e., a low loss when the surface wave islaunched and transmitted. Signals lying outside this band should bedissipated in the filter.

A narrow band filter can, for example, be obtained by increasing thenumber of electrode fingers, so that a longer transducer is obtained.

In a standard finger transducer, the electrode fingers that areconnected to different bus bars have a finger center separation ofλ/2.In the conversion of a standard finger transducer to a split fingertransducer, two split fingers arranged at a separation of λ/4 replaceeach electrode finger of the standard finger transducer. The two splitfingers are inherently mechanically reflection-free, since thereflections of the two fingers cancel one another out. But, even in thiscase, problems can still occur with longer transducers, causing a splitfinger transducer not to be reflection-less due to non-zero electricalregeneration at the terminal impedance at the acoustic gates.

SUMMARY

An article by P. Dufilie and P. Ventura, entitled “Source Equalizationfor SPUDT Transducers” in IEEE Ultrasonics Symposium 1995, pp. 13-16,provides known rules for creating a transducer with distributed acousticreflection, or a so-called DART transducer. A DART transducer havingunidirectional characteristics is a SPUDT transducer (SINGLE PHASEUNIDIRECTIONAL TRANSDUCER). In this type of transducer, exciting andreflecting electrode fingers are different. A reflection-free pair ofsimilar electrode fingers with a finger center separation of λ/4 is usedfor excitation. In a unit cell with length λ, there is also a reflectingelectrode finger. By adjusting the width and exact position of thereflecting electrode transducer, the transducer reflection can beadjusted. In this manner, it is possible to model a transducer that hasa desired reflection distributed over the transducer. This distributedreflection can be weighted.

The object of this invention is to provide a transducer with distributedexcitation and reflection that has a high unidirectionality andsymmetric electroacoustic conversion with respect to a center frequency.

This object is met, according to the invention, by a transduceraccording to claim 1. Advantageous embodiments of the invention can befound in the dependent claims.

Starting from a transducer having known DART cells, the transduceraccording to the invention also has a distributed reflection, but doesnot have the limitations of the DART cell. Whereas the latter requires astrict separation of the exciting electrode finger pair and thereflector fingers, the transducer of the present invention does notrequire such a strict separation. A transducer according to theinvention is built from a number “n” of base cells, arranged one afterthe other in the direction of propagation of a surface wave. Altogetherthe cells have an approximate length of λ, which is the center frequencyof the transducer. The transducer can be split up into exciting andreflecting base cells, in which the reflection portion has only specificvalues m·R_(o), where “m” can have values −2, −1, 0, 1, or 2, and whereR_(o) is a reference reflection. Each reflection portion that is not 0in value has the same phase position φ₀. Phase position and excitationstrength are identical in all exciting base cells, as is the number ofexciting fingers. The phase relationship between excitation andreflection results in unidirectional behavior in the transducer, withphase equality being maintained in one preferential direction, and phaseopposition being maintained in the opposite direction.

The subject transducer is no longer strictly subdivided into excitingand reflecting fingers. Instead, exciting fingers also contribute areflection portion that is optimized to the desired phase position andstrength by varying the finger width and finger position. This alsoincreases the unidirectionality of the transducer, which leads to adecreased insertion loss, a longer impulse response, and steeper sidesof the band pass of the band pass curve in a filter containing atransducer of this type.

All of the exciting base cells of the transducer can each have exactlyone electrode finger individually connected to a bus bar as the excitingelectrode finger, thus forming single finger cells, or so-called EWCcells. Dual finger cells are also possible. In dual finger cells, allexciting base cells have one group of two electrode fingers each,connected to a common bus bar, as exciting electrode fingers. In thesecells, the exciting electrode fingers in each group have differentfinger widths and a finger center separation that always differs fromλ/4. Advantageously, the dual finger cells produce a relatively largeexcitation per exciting base cell. In the single finger cells, on theother hand, in most cases a larger reflection portion of the excitingbase cell can be maintained as a function of the minimal structurewidth. Finally, the transducer is preferred to have only one type ofexciting base cell, since all exciting cells have the same excitationstrength. This is the case for both single and dual finger cells.

One excitation center and/or one reflection center can be determined ineach base cell of the transducer. In base cells according to theinvention or a transducer according to the invention, the separation ofthe excitation centers from the reflection centers is 3λ/8. Thisseparation applies to all base cells that provide excitation andreflection portions.

In the transducer, the reflection strength can be maximized in thereflecting base cells. This does not mean that all cells have maximumreflection, but that the reference reflection strength R_(o) is set to amaximum value to which all base cells can correspond with the exceptionof the non-reflecting cells. The reflection strength of the base cellthat has the lowest reflection strength after optimizing all base cellsto maximum reflection strength then serves as reference R_(o).

In another embodiment of the transducer, the finger widths and theseparations of the electrode fingers continuously increase or decreasein the transverse direction (perpendicular to the direction ofpropagation of the surface wave). This type of modification increasesthe bandwidth of a transducer and thus the bandwidth of a filter inwhich the transducer is used.

In another embodiment of the invention, the transducer is designed to befocusing and has electrode fingers with bent edges. A transducer of thistype effects a reduction in leakage losses when used as an inputtransducer in a surface acoustic wave filter. By being focused, eventhose surface waves that in an input transducer with straight electrodefingers would no longer reach the output transducer make their way tothe receiving or output transducer. This also lowers the insertion lossof the transducer and the filter.

In each base cell, in general, all finger widths and all fingerseparations of the electrode fingers are different. This means thatwithin a base cell a specific maximum finger width or a specific maximumfinger separation occurs only once.

Due to the advantageous properties of the transducer, it may be used inan IF filter that has a low insertion loss and that has a longer impulseresponse due to the additional resonance spaces created.

A method to determine optimal transducer geometries is described in moredetail below with respect to an exemplary embodiment and its associatedfigures. These show:

DESCRIPTION OF THE DRAWINGS

FIGS. 1a through 1 c show three different approaches for single fingercells.

FIGS. 2a through c show different types of dual finger cells.

FIG. 3 shows an exemplary base cell according to the invention.

DETAILED DESCRIPTION

The precise determination of the finger geometries, in particular thefinger widths and the finger separations, is made by formulating asolvable optimization task. Optimization procedures for transducers areknown, but are subject to limitations such that the cells to beconstructed have lesser degrees of freedom than available geometricwidths. For an optimization procedure to produce transducers accordingto the invention, the problem can be generalized so that restrictionsthat apply to previous optimization procedures, in particular the fixedrelationships between finger widths and positions, no longer apply.Until now, weighting the reflection of a cell was only possible byadjusting the metallization height. According to the invention, it isnow possible to continuously vary the widths of the reflection finger orfingers.

The non-exciting geometries are directly formed from the geometries withexcitation by leaving out the overlap, e. g., by modifying the fingerconnection sequence. Starting geometries of the base cells for theoptimization task are selected from known single and dual finger cells.

FIGS. 1a through c show three different approaches for single fingercells, in which there is only one electrode finger at eachsignal-carrying bus bar. The single finger cells can be comprised ofthree or four electrode fingers.

FIG. 1a shows a single finger cell without reflection that can be usedas an output point. The single finger cell has a regular λ/8 fingerarrangement in a λ/2 grid and is reflection-less when electricallyshort-circuited.

FIG. 1b shows a single finger cell with positive reflection that has anadditional reflecting electrode finger with a width 3λ/8 in addition toan electrode finger pair with finger width and finger separation=λ/8.

FIG. 1c shows how a single finger cell with negative reflection arisesfrom a cell by exchanging two electrode fingers. The phase difference ofthe reflection between cells with positive reflection and cells withnegative reflection is 90° such that the phase difference of reflectionat the ends of this single finger transducer is 180°.

FIGS. 2a through c show different types of dual finger cells, in whichtwo electrode fingers are connected to signal-carrying bus bars.

FIG. 2a again shows a regular λ/8 finger arrangement that isreflection-free.

FIG. 2b shows a dual finger cell with positive reflection.

FIG. 2c shows a dual finger cell with negative reflection.

The cited, known single and dual finger cells are used as a startingpoint for the optimization process. If a transducer is electricallyconnected, i. e., is connected to an external load, a regenerationsignal arises due to acoustic electrical feedback of the surface wave tothe electrode fingers. This signal influences the behavior of thetransducer. A transducer that is reflection-free under load can beoptimized such that the reflected portions of the wave can exactlycancel the regeneration signal. To accomplish this, a suitable phaserelationship and corresponding amplitude ratio must be produced.However, other optimization goals are also conceivable for transducersaccording to the invention for specific applications.

In the optimization process to determine a final transducer geometry,all of the foregoing points are taken into account and lead, in the end,to a transducer in which the electrode fingers have different fingerwidths and finger separations within its base cells. This is also aresult of iterative minimization of the phase error that is requiredafter optimization.

FIG. 3 shows a reflecting dual finger cell of a transducer resultingfrom an optimization process. Each cell has two dual fingers with afinger width of 0.0829*λ and 0. 1004* λ, respectively, at a fingerseparation of 0.1229*λ. A considerable improvement in the transferfunction has been shown with the transducer compared to knowngeometries. The attenuation is clearly increased, the sides are steeperand the insertion loss is reduced.

What is claimed is:
 1. A transducer comprising: base cells each having alength λ, where λ is a wavelength corresponding to a center frequency ofthe transducer, the base cells being arranged in a direction ofpropagation of a surface acoustic wave, the base cells includingexciting base cells, the base cells including an excitation portion anda reflection portion, the excitation portion being non-zero for excitingbase cells, the reflection portion of each base cell having a strengthmR₀, where m is one of −2, −1, 0, 1, or 2 and R₀ is a referencereflection; wherein the base cells include electrode fingers, a width ofthe electrode fingers is not a multiple of λ/16, and a separationbetween adjacent electrode fingers is not a multiple of λ/8; wherein, inthe direction of propagation, a non-zero reflection portion in each basecell has a same phase position φ±πn, where n is an integer greater thanzero; wherein centers of excitations of base cells having a same signare separated by 2πn and centers of excitations of base cells havingdifferent signs are separated by (2n−1)π, and wherein surface acousticwaves excited and reflected in at least one base cell are equal in phasein the direction of propagation and opposite in phase in a directionthat is opposite to the direction of propagation.
 2. The transduceraccording to claim 1 wherein in at least one base cell, one electrodefinger is connected to a first bus bar and all other electrode fingersare connected to a second bus bar, the one electrode finger having awidth that is not λ/8, the one electrode finger having a separation froman adjacent electrode finger that is not λ/4 or 3λ/8.
 3. The transduceraccording to claim 1 wherein groups of two electrode fingers areconnected to a common bus bar, one group each per exciting base cell,and wherein electrode fingers in a group of two electrode fingers eachhave a different finger width that is not λ/16 or 3λ/16 and a fingercenter separation that is not λ4.
 4. The transducer according to claim 2wherein, in the exciting base cells, a separation between excitationcenters and reflection centers is 3λ/8.
 5. A transducer according toclaim 1 wherein electrode finger widths and separations increase intransverse direction.
 6. A transducer according to claim 1 wherein theelectrode fingers have bent edges.
 7. A transducer according to claim 1wherein electrode finger widths and separations are different in eachbase cell.
 8. An IF filter with low insertion loss and long impulseresponse, which includes the transducer of claim
 1. 9. The transducer ofclaim 1 wherein excitations of all exciting base cells have asubstantially identical strength.
 10. The transducer of claim 1 whereina number of electrode fingers connected to first and second bus bars isidentical for all base cells having a non-zero excitation portion.